New stretchable, biocompatible materials with complex patterning could be used for creating a human nose or ear

June 5, 2015

Lasagna? No, an open lattice of 3-D printed material, with materials having different characteristics of strength and flexibility indicated by different colors (credit: the researchers)

Researchers at three universities have developed a new way of making tough — but soft and wet — biocompatible hydrogel materials into complex and intricately patterned shapes. The process might lead to scaffolds for repair or replacement of load-bearing tissues, such as cartilage. It could also allow for tough but flexible actuators for future robots, the researchers say.

Zhao says the process can produce complex hydrogel structures that are “extremely tough and robust,” but still allow for encapsulating cells in the structures. That could make it possible to 3D-print complex biostructures.

Biocompatible structures

Hydrogels are defined by water molecules encased in rubbery polymer networks that provide shape and structure. They are similar to natural tissues such as cartilage, which is used by the body as a natural shock absorber.

While synthetic hydrogels are commonly weak or brittle, a number of them that are tough and stretchable have been developed over the last decade. However, making tough hydrogels has usually involved “harsh chemical environments” that would kill living cells encapsulated in them, Zhao says.

The new hydrogel materials are generated by combining polyethylene glycol (PEG) and sodium alginate, which synergize to form a hydrogel tougher than natural cartilage. The materials are benign enough to synthesize together with living cells — such as stem cells — which could then allow high viability of the cells, says Zhao, who holds a joint appointment in MIT’s Department of Civil and Environmental Engineering.

Previous work was not able to produce complex 3-D structures with tough hydrogels, Zhao says. The new biocompatible tough hydrogel can be printed into diverse 3-D structures such as a hollow cube, hemisphere, pyramid, twisted bundle, multilayer mesh, or physiologically relevant shapes, such as a human nose or ear.

The new method uses a commercially available 3D-printing mechanism, Zhao explains. “The innovation is really about the material — a new ink for 3-D printing of biocompatible tough hydrogel,” he says, specifically, a composite of two different biopolymers.

“Each [material] individually is very weak and brittle, but once you put them together, it becomes very tough and strong. It’s like steel-reinforced concrete.”

The PEG material provides elasticity to the printed material, while sodium alginate allows it to dissipate energy under deformation without breaking. A third ingredient, a biocompatible “nanoclay,” makes it possible to fine-tune the viscosity (how easily it flows) of the material, improving the ability to control its flow through the 3D-printing nozzle.

The material can be made so flexible that a printed shape, such as a pyramid, can be compressed by 99 percent, and then spring back to its original shape, Sungmin Hong, a lead author of the paper and a former postdoc in Zhao’s group, says; it can also be stretched to five times its original size. Such resilience is a key feature of natural bodily tissues that need to withstand a variety of forces and impacts.

Such materials might eventually be used to custom-print shapes for the replacement of cartilaginous tissues in ears, noses, or load-bearing body joints, Zhao says. Lab tests have already shown that the material is even tougher than natural cartilage.

Enhancing resolution

The next step in the research will be to improve the resolution of the printer, which is currently limited to details about 500 micrometers (0.5 millimeters) in size, and to test the printed hydrogel structures in animal models. “We are enhancing the resolution,” Zhao says, “to be able to print more accurate structures for applications.”

The technique could also be applied to printing a variety of soft but tough structural materials, he says, such as actuators for soft robotic systems.

“This is really beautiful work that demonstrates major advances in the utilization of tough hydrogels,” says David Mooney, a professor of bioengineering at Harvard University who was not involved in this work. “This builds off earlier work using other polymer systems, with some of this earlier work done by Dr. Zhao, but the demonstration that one can achieve similar mechanical performance with a common biomedical polymer is a substantial advance.

“It is also quite exciting that these new tough gels can be used for 3-D printing, as this is new for these gels, to my knowledge.”

The work was supported by the National Institutes of Health, the Office of Naval Research, AOSpine Foundation, and the National Science Foundation.

A 3D printable and highly stretchable tough hydrogel is developed by combining poly(ethylene glycol) and sodium alginate, which synergize to form a hydrogel tougher than natural cartilage. Encapsulated cells maintain high viability over a 7 d culture period and are highly deformed together with the hydrogel. By adding biocompatible nanoclay, the tough hydrogel is 3D printed in various shapes without requiring support material.

comments 2

One big disadvantage of using PEG for biological tissue scaffolding is that it is not biodegradable, however there is still some possibilities to explore about this, see e.g. http://tinyurl.com/pjxhr24